For therapy of allergic diseases such as allergic asthma, allergic rhinitis, allergic dermatitis and allergic conjunctivitis, an anti-allergic or anti-histaminic agent is used in general. This therapy, however, being a symptomatic treatment, is unable to radically cure allergic diseases. Hyposensitization where causative antigens of allergic diseases are injected is believed to be a sole measure for radical cure of allergic diseases. Hyposensitization is performed by injecting to a patient suffering from allergic diseases a small amount of allergens causative for said allergic diseases so that allergic reactions within the living body might disappear. Its efficacy is more than 70%.
Sole mite allergen preparations currently used are house dust allergen preparations, which are prepared by the process of extracting allergens from house dust. The mite allergen preparations prepared by said process thus also contain a variety of antigens such as e.g. mould or bacteria in addition to mite thereby not to allow for unambiguously defining their biological activity (titer). There is also a problem that stable provision of a large quantity of allergens is difficult due to a limited source material therefor.
Allergen preparations are mainly administered subcutaneously for immune induction. If allergen preparations are contaminated with impurities, however, the living body receiving said preparations will possibly induce an immune response to the undesired impurities to thereby establish sensitization thereto. Thus, there is possibility that allergy to impurities is newly induced to cause serious adverse side effects. Accordingly, to ensure that allergen preparations are safe, highly purified preparations need be provided.
However, even if allergen preparations are highly purified, there is still a risk that administering the preparations causes anaphylactic shock. It is known that type I allergy, typically anaphylactic shock, is caused when allergens bind IgE antibody and hence administration of allergens always involves some risk of causing anaphylactic shock.
In recent years, with progress of study in mite allergens, several major mite allergens have already been identified (e.g. Platts-Mills et al., J. Allergy Clin. Immunol. 80, 755-775, 1987; incorporated herein by reference). For instance, Yuhki et al. disclosed a gene encoding major mite allergen Der f 2 and a process for preparing the same (Japanese J. Allergology, vol. 39, p. 557, 1990; incorporated herein by reference). Hyposensitization for therapy of allergic diseases using this major mite allergen might be quite efficacious solving the problems as described above but is not able to completely exclude risk of anaphylactic shock possibly caused by administration of allergens.
A modified type of a major mite allergen (hereinafter also referred to as “modified major mite allergen”) has been prepared using the genetic recombination technique wherein a steric structure of mite allergen is partly destructed so as to reduce its binding to IgE antibody (e.g. Japanese Patent Publication No. 253851/1994 (Japanese Patent Application No. 139793/1993); incorporated herein by reference). This modified major mite allergen, resulting from replacement of the cysteine residue in major mite allergen Der f 2 with the serine residue, is expected to be useful as a medicament for therapy of allergy by safely inducing hyposensitization without causing anaphylactic shock. However, there is no report that major mite allergens produced by the genetic engineering process on an industrial level are purified to a pharmaceutical grade.
A recombinant major mite allergen Der f 2 has been prepared by transforming prokaryotes or eukaryotes with an expression vector comprising a gene encoding Der f 2, culturing the transformants and purifying Der f 2 from the culture. According to the process disclosed in Japanese Patent Publication No. 253851/1994 as described above, purification from culture is performed by ion exchange chromatography and gel filtration chromatography. However, the thus purified Der f 2 has purity of merely a reagent grade. Moreover, according to the process, refolding of the modified mite allergen is performed by dialysis and hence treatment in a large quantity is difficult.